In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and printing ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (also referred to as a photosensitive layer, an image-recording layer or an image-forming layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-recording layer corresponding to the image area, removing other unnecessary image-recording layer by dissolving with an alkaline developer or an organic solvent to reveal the hydrophilic surface of support thereby forming the non-image area.
With the developments of image-forming technology in recent years, a computer-to-plate technique in which plate making is performed by directly exposing a lithographic printing plate precursor with laser based on digitalized data, for example, from a computer without using a lith film has been developed and high-sensitive lithographic printing plate precursors of laser recording system for use in the technique have also been developed.
On the other hand, instead of the process of removing the unnecessary image-recording layer by dissolution with the alkaline developer or the like after the image exposure, a method of obtaining a lithographic printing plate by processing with a developer closer to neutral region or a method referred to as on-press development in which after the image exposure, the non-image area is removed on a printing machine to obtain a lithographic printing plate has been proposed.
In order to respond to these image exposure and development processing, various developments have been made. In particular, for the purpose of improving printing durability and stain resistance of lithographic printing plate, a large number of proposals with respect to a lithographic printing plate precursor and a processing method have been made. For example, in JP-A-2005-238816 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), with the intention to the improvements in the printing durability and the like, a technique of incorporating a low molecular weight compound having a functional group capable of interacting with the surface of support into an image-forming layer or an undercoat layer provided on the support is proposed and as examples of the low molecular weight compound having a functional group capable of interacting with the surface of support, compounds having a group connecting a support-adsorbing group through a hetero atom in an α-position of an ethylenically unsaturated bond are described. Also, in JP-A-2006-239860, JP-A-2006-239867 and JP-A-2006-264333, with the intention to the improvements in the printing durability and the like, techniques of incorporating a copolymer containing a repeating unit having an ethylenically unsaturated bond and a repeating unit having a functional group capable of interacting with the surface of support into an image-forming layer or an undercoat layer provided on the support are proposed and copolymers containing as the repeating unit having an ethylenically unsaturated bond, a repeating unit having a group connecting the ethylenically unsaturated bond through a hetero atom in its α-position are described.
According to these prior arts, although the printing durability and the like are improved, further improvements have been desired. In particular, a lithographic printing plate precursor which can prepare a lithographic printing plate excellent in stain resistance as well as the printing durability and which is prevented from deterioration of the stain resistance even when the lithographic printing plate precursor is preserved has been desired.